If you have done some experimenting with bookbinding materials, you might have considered using cereal box card. The high acidity level of this type of board for a book spine makes it less than ideal, but there are ways you can rectify that.

It is possible to reduce the acidity in cereal box card by soaking it in calcium carbonate and drying it under weight, between blotters. You can also coat it with PVA to seal in the board, reducing contact with the air.

However these methods are time consuming and use a lot of bookbinding glue if you choose the PVA method, so it will cost you more ultimately than buying good board.

You can also purchase testing kits to test the acidity of your bookbinding board. Litmus paper also works where you wet out a sample of the chosen material and lay the litmus paper on it. This paper gives you a basic acid vs. base reading by its color.

So in conclusion, cereal box cardboard is not a good bookbinding material for using in book spines. It is not structurally stable enough to stay flat even when surrounded by paper on both sides.